1. Field of the Invention
Activated carbons are high porosity, high surface area materials used in industry for purification and chemical recovery operations as well as environmental remediation. Toxic metals contamination of various water sources is a significant problem in many parts of the United States. Activated carbons, which can be produced from a number of precursor materials including coal, wood and agricultural wastes, are now being actively utilized for remediation of this problem. Carbon production is an expanding industry in the United States, with a present production rate of over 300 million pounds a year and a growth rate of over 5% annually. The present invention relates to the development of specifically modified granular carbons from agricultural waste products that possess enhanced adsorption properties with regard to the uptake of metal ions.
2. Description of the Prior Art
The production of carbon, in the form of charcoal, is an age-old art. Carbon, when produced by non-oxidative pyrolysis, is a relatively inactive material possessing a surface area limited to several square meters per gram. In order to enhance its activity, a number of protocols have been developed. These include chemical treatment of the carbonaceous material with various salts or acids prior to pyrolysis, or a reaction of the already pyrolyzed product with high temperature steam. Activated carbon is able to preferentially adsorb organic compounds and non-polar materials from either liquid or gaseous media. This property has been attributed to its possession of a form which conveys the desirable physical properties of high porosity and large surface area.
Whitehead et al., in a paper entitled "Studies in the Utilization of Georgia Pecans", (State Engineering Experiment Station Bulletin, The Georgia School of Technology, Vol.1, No.5, December 1938, pp. 3-11), disclose the production of activated charcoal by treating pecan hulls with hydrochloric acid and then heating in an atmosphere of carbon dioxide for four hours at a temperature ranging from 800-1000.degree. C. This product was described as having the same decolorizing power on water solutions of azo dyes as commercially available activated charcoals.
Bevia et al., in an article entitled "Activated Carbon from Almond Shells. Chemical Activation. 1. Activating Reagent Selection and Variables Influence" (Ind. Eng. Chem. Prod. Res. Dev. 1984, 23, 266-269), discuss the preparation of activated carbon from almond shells. The activating chemicals H.sub.3 PO.sub.4, ZnCl.sub.2, K.sub.2 CO.sub.3, and Na.sub.2 CO.sub.3 were utilized in the study, with products derived from activation by ZnCl.sub.2 giving the best results. It was further found that the impregnation ratio (activating reagent/raw material) was the most critical parameter, with materials made at ratios higher than 100% giving the best products.
Jagtoyen et al., in their paper entitled "Some Considerations of the Origin of Porosity in Carbons from Chemically Activated Wood", (Carbon, Vol. 31, No. 7, pp.1185-1192, 1993), investigated the conversion of white oak to activated carbons by reaction with phosphoric acid and subsequent heat treatment under nitrogen to temperatures ranging from 50.degree. C. to 650.degree. C. They found that the carbon structures created undergo significant expansion, with an accompanying development of high surface area, at reaction temperatures ranging from 250.degree. C. to 450.degree. C. At reaction temperatures above 450.degree. C. there is secondary product contraction with an associated loss of product porosity. From this evidence it was concluded that porosity development is directly related to the retention and dilation of cellular material.
Molina-Sabio et al., in their paper entitled "Modification in Porous Texture and Oxygen Surface Groups of Activated Carbons by Oxidation", (Characterization of Porous Solids II, Rodriguez-Reinoso et al., [edit.] 1991 Elsevier Science Publishers B.V., Amsterdam), disclose that while oxidation treatment of fruit pits by either air or chemical means (HNO.sub.3 or H.sub.2 O.sub.2) does not substantially modify the microporosity of the carbon structures created, the chemical nature of the carbon surface is changed considerably. No projected uses for these carbons are set forth.
Molina-Sabio et al., in a publication entitled "Influence of the Atmosphere used in the Carbonization of Phosphoric Acid Impregnated of Peach Stones" (Carbon, pp. 1180-1182, 1995), teach that the inclusion of air during the heating step of the acid activation of carbons should not result in any appreciable reaction with the carbon material. This is premised upon the fact that there is a continuous evolution of decomposition gases during the activation process.
Periasamy et al., in an article entitled "Process Development for Removal and Recovery of Cadmium from Wastewater by a Low-Cost Adsorbent: Adsorption Rates and Equilibrium Studies", (Ind. Eng. Chem. Res., 33, 317-320, 1994), show that at a concentration of 0.7 g/L, activated carbon produced from peanut hulls was able to achieve an almost quantitative removal of Cd(II) present at a concentration of 20 mg/L in an aqueous solution at a pH range of 3.5-9.5.
Balci et al., in their article "Characterization of Activated Carbon Produced from Almond Shell and Hazelnut Shell", (J. Chem. Tech. Biotechnol., 1994, 60, 419-426), show that chemical activation of ammonium chloride-impregnated almond and hazelnut shell at 350.degree. C. and 700.degree. C. gave products with surface area values in excess of 500 m.sup.2 /g and 700 m.sup.2 /g respectively. These values were approximately twice that observed for products derived from untreated raw materials.
Moreno-Castilla et al., in an article entitled "Activated Carbon Surface Modifications by Nitric Acid, Hydrogen Peroxide, and Ammonium Peroxydisulfate Treatments" (Langmuir, 1995, 11, 4386-4392), disclose the principle that acidic oxygen surface complexes are formed on activated carbons as a result of their treatment with either gas or solution phase oxidizing agents; and that inclusion of these complexes effect changes in the behavior of activated carbons when used either as adsorbents or catalysts.
Rivera-Utrilla et al., in their publication entitled "Effect of Carbon--Oxygen and Carbon--Nitrogen Surface Complexes on the Adsorption of Cations by Activated Carbons", (Adsorption Science & Technology, 1986, 3, 293-302), disclose that activated carbons obtained from almond shells are capable of removing trace amounts of various metal ions from aqueous solutions.
While various methodologies for the creation of granular activated carbons exist, there remains a need for the creation of alternate viable and cost-effective products possessing enhanced adsorption characteristics.